The present invention relates to a superconducting quantum interference device (hereinbelow abbreviated to SQUID) suitable for detecting very weak magnctic flux, and in particular to a SQUID utilizing high temperature superconductor.
The SQUID is a magnetic fluxmeter having the highest sensitivity, utilizing the quantum interference effect, which electrons in the superconductive state have. For this reason, SQUIDs are used as devices for measuring very weak magnetic field, e.g. for measurement of magnetic field in a living body, measurement of terrestrial magnetism, search of underground resources, etc.
The SQUID has an excellent performance as a fluxmeter. However, since the operating temperature of a Josephson element is restricted by the critical temperature of the superconductor, the SQUID using a prior art superconductor made of metal such as Nb, etc. was used at the liquid helium temperature (boiling point: absolute temperature 4.2 K). However, since liquid helium is expensive and further because of problems concerning cryogenic techniques, there existed restrictions in its application field. Recently, owing to the discovery and development of oxide high temperature superconductors the application of the SQUID has entered a new stage. Attention has been paid to SQUIDs using liquid nitrogen (boiling point: absolute temperature 77 K) which is cheap and used widely as cooling medium, which has been realized by using an oxide high temperature superconductor. However, since the operating temperature of the SQUID using an oxide high temperature superconductor is 77 K, which is about 20 times as high as the operating temperature of the prior art SQUID, which is 4.2 K, there exist technical problems, which are to be solved.
A SQUID using a high temperature superconductor thin film is disclosed in an article published in APPl. Phys. Lett., 51, (1987) pp 200-202. Although some examples on the SQUID using a high temperature superconductor thin film have been published, it has been not yet realized to study technical problems for the purpose of practical use and to find solutions therefor.